1. Field of the Invention
The invention relates to a hydrostatic drive system for a mobile machine, such as a fork lift truck, with a travel drive and working hydraulics. The working hydraulics have a variable delivery pump which is operatively connected to the driving engine, is connected to a container on the inlet side, and delivers to a delivery line, to which at least one consumer of the working hydraulics is connected. The consumer is controllable by means of a control valve device having a closed neutral position. The pump is adjustable in the direction of a higher delivery to generate a braking action in a braking or overrun condition of the travel drive and the pressure medium delivered by the pump is feedable to a pressure valve.
2. Technical Considerations
Mobile machines, for example fork lift trucks, with a driving engine designed as an internal combustion engine, in particular a diesel engine, generally have a hydrostatic travel drive with at least one motor connected in a closed circuit to a pump, and working hydraulics operated in an open circuit. The pump of the working hydraulics may be designed as a variable delivery pump. The control valve devices for controlling the consumers of the working hydraulics, for example a lifting drive, an inclining drive, and, optionally, a side loader, as well as further additional consumers of a machine designed as a fork lift truck, in this case have a closed neutral position in which the delivery line of the pump is closed off.
The hydrostatic travel drive may be used as a service brake. In a braking condition, the driving engine is adjusted to the lower idling rotational speed and the transmission ratio of the travel drive is changed, for example by adjusting the pump in the direction of minimum delivery. As a result of the mass inertia of the vehicle, the power flow in the closed circuit reverses, the motor operating as a pump and delivering pressure medium to the pump operating as a motor. In overrun condition, for example when the machine is travelling downhill, the motor is likewise driven and operates as a pump, which delivers pressure medium to the pump operating a motor.
In the pressure-medium line, led from the motor to the pump, of the travel drive, a braking pressure builds up in braking or overrun condition and generates at the pump a torque which accelerates the driving engine. The drag torque which can be taken up by the driving engine can thus be used to brake the machine.
The increase in rotational speed of the driving engine which occurs in braking or overrun conditions may lead to racing of the driving engine at undesirably high rotational speeds if the drag torque, which can be taken up by the driving engine, is not sufficient to brake the vehicle. As a result of which, mechanical damage and increased wear of the driving engine may occur.
In order to avoid such an increase in rotational speed of the driving engine in braking or overrun conditions to an undesirably high rotational speed, it is known to provide an additional braking action by using the pump provided for supplying the working hydraulics to consume braking energy as well. In drive systems with a variable delivery pump for supplying the working hydraulics, the pump is in this case adjusted in the direction of a higher delivery in a braking or overrun condition, and the pressure medium accumulated in the delivery line of the pump is drained to a container via a pressure valve designed as a pressure-limiting valve and arranged in a line connecting the delivery line of the pump to the container.
In such an arrangement of the pressure valve, in which the pressure medium delivered by the pump in a braking or overrun condition is drained to a container via the pressure valve, operating states may, however, occur in which no pressure medium is supplied to the consumers in a braking or overrun condition of the machine. As a result of which, no actuation of the consumers of the working hydraulics is possible and thus no functions of the working hydraulics can be carried out. Particularly in the case of a machine designed as a fork lift truck, however, in a braking or overrun condition, in particular in a reversing operation, i.e., braking and change of the direction of travel, an actuation and, thus, a function of the consumers of the working hydraulics is desired to enable a movement of the load picked up by the fork lift truck.
An object on which the present invention is based is to provide a drive system of the generic type mentioned above but which ensures an actuation of the consumers of the working hydraulics in an overrun or braking condition.